I am redesigning the whole site, using MAMPP and a localhost, and Ultimatum theme builder to create the WP site. My new website will have pages, photo galleries, and the already existing blog articles.

My main domain will remain the same, but I will redirect the old .asp pages. To avoid changing the url:s of my blog pages, I was thinking to keep the blog on a subdomain and, while creating the new pages and the gallery-category-posts on my main domain.
How would I go about doing that?

Would I need to have a WP multi-site with one subdomain? It seams unnecessary complicated when I will not have more sites or anyone else creating sites…

Or would it be easier to import my blog to my main site and then use a plug-in to assign all blog posts to the original subdomain. But doing so, would the page_id of each post change, so that I would have to set up re-diects for the old posts anyway?

Sorry it’s a very specific “my case” question, but I find so many options regarding creating subdomains, redirecting plugins, and transferring WP sites, that I need some advice to get started.